was used to compute the expected number of com- 

 mon dolphins in that class. 



The distribution of pilot whales was not the same 

 as that of common dolphins among the CI classes (P 

 < 0.005), particularly in the class for the lowest 

 relief. Of the total chi-square value, 63% is due to 

 proportionally more observations of dolphins than 

 pilot whales in this one class (Table 2). 



The distributions of the two sets of survey flights 

 used to collect data for the two species were com- 

 pared, but the CI range in this case was divided into 

 10 equal classes. The two sets of survey flights were 

 equally distributed among the CI classes {P > 0.10). 



Table 2. — Comparative distribution among contour index 

 classes of pilot whales and common dolphins. 



'Computed from pilot whale sightings; see text. 



Discussion 



In the California Bight, pilot whales and common 

 dolphins are distributed similarly above undersea 

 topography of high relief, but common dolphins oc- 

 cur more frequently than pilot whales over areas of 

 low relief. This difference may be linked to differ- 

 ences in feeding habits. 



Pilot whales and common dolphins have significant 

 morphological and physiological differences that are 

 associated with their foods and feeding methods. 

 Pilot whales are larger (7 m vs. 2 m in length) and 

 have fewer but larger teeth (40 vs. 200) than com- 

 mon dolphins (Orr 1972). Pilot whales are capable of 

 diving to 610 m (Bowers and Henderson 1972) while 

 common dolphins dive to 257 m (Evans 1971). 



The diet of the common dolphin in the Southern 

 California Bight includes 19 species of fish, 2 species 

 of squid, and miscellaneous crustaceans (Fitch and 

 Brownell 1968; Evans 1975). 



In contrast to common dolphins, pilot whales ap- 

 pear to be stenophagous, eating primarily squid. 

 Atlantic pilot whales, GLobicephala melaena, eat 

 primarily squid {Illex illecebrosus). The only fish 

 reported eaten (cod, Gadus morhua) composes <10% 

 of the diet (Sergeant 1962). There has been no study 

 on the Pacific pilot whale comparable with that of 

 Sergeant's (1962) on the Atlantic form; however, the 



stomachs of four wild Pacific pilot whales have been 

 examined. They contained squid but no fish (W. A. 

 Walker,! (] \y Woodhouse,^ D. J. Seagars^). In cap- 

 tivity an Atlantic pilot whale rejected mullet (Mugili- 

 dae) and blue runner (Carangidae) fish and accepted 

 only squid (probably Loligo pealei) until it was trick- 

 ed into eating some herring (Clupeidae); and then the 

 indications were that "He did not seem to digest the 

 fish as well" (Kritzler 1949). 



Squid distribution cannot be related to any particu- 

 lar bottom topography along the California coast 

 because squids are not easily collected with sampling 

 methods used in distribution studies (Mais 1974). 

 However, it may be inferred that the narrow range 

 of seafloor topographies visited by pilot whales 

 reflects the narrow range of their diet and the areas 

 where squid can be most easily caught by pilot 

 whales. 



Also concentrated over areas of canyons and 

 escarpments are anchovies (Mais 1974), a major com- 

 ponent of the dolphin diet (Fitch and Brownell 1968; 

 Evans 1975). Common dolphins frequent these areas 

 most. However, common dolphins are euryphagous. 

 Some prey probably occur over seafloor of low relief, 

 although this could not be confirmed from fish 

 reports because bottom topography is not a para- 

 meter which is recorded in fish distribution studies. 

 If some prey do occur over areas of low relief, their 

 distribution would partially explain why dolphins oc- 

 cur over seafloor of low relief more frequently than 

 do pilot whales. 



My analyses show that the daytime distribution 

 patterns of these two pelagic cetacean species are 

 not random but are related to bottom topography. 

 Although the distributions are similar, they are not 

 the same. Differences in distributions may be due to 

 the different foraging patterns but no firm conclu- 

 sion can be drawn until more information is 

 available. 



Acknowledgments 



I thank G. A. Bartholomew, M. F. Platter-Rieger, 

 F. G. Wood, and two anonymous reviewers for their 

 helpful comments; also D. J. Seagars, W. A Walker, 

 and C. W. Woodhouse for information on the 



'W. A. Walker, Research Associate, Section of Mammalogy, 

 Natural History Museum of Los Angeles County, Los Angeles, CA 

 90007, pers. commun. July 1980. 



-C. W. Woodhouse, Curator of Vertebrate Department, Santa 

 Barbara Museum of Natural History, 2559 Puesta del Sol Road, 

 Santa Barbara, CA 93105, pers. commun. July 1980. 



'D. J. Seagars, Wildlife Biologist, National Marine Fisheries Ser- 

 vice, Southwest Region, 300 S. Ferry St., Terminal Island, CA 

 90731, pers. commun. January 1981. 



474 



